Symfony Dependency Injection. Create a custom config loader.

AppKernel.php

<?php

use Symfony\Component\Config\Loader\LoaderInterface;
use Symfony\Component\HttpKernel\Kernel;

class AppKernel extends Kernel
{
    public function registerBundles()
    {
        // ...
        
        return $bundles;
    }
    protected function getContainerLoader(\Symfony\Component\DependencyInjection\ContainerInterface $container)
    {
        $delegating = parent::getContainerLoader($container);

        /** @var \Symfony\Component\Config\Loader\LoaderResolver $resolver */
        $resolver = $delegating->getResolver();

        $resolver->addLoader(new \AppBundle\DependencyInjection\Loader\MyCustomConfigLoader($container));

        return $delegating;
    }
}

config.yml

imports:
    - { resource: my_custom_loader }

CustomConfigLoader.php

<?php

namespace AppBundle\DependencyInjection\Loader;

use Symfony\Component\Config\Loader\Loader;
use Symfony\Component\DependencyInjection\ContainerBuilder;

class MyCustomConfigLoader extends Loader
{
	/**
	 * @var ContainerBuilder
	 */
	protected $container;

	/**
	 * MyCustomConfigLoader constructor.
	 *
	 * @param \Symfony\Component\DependencyInjection\ContainerBuilder $container
	 */
	public function __construct(ContainerBuilder $container)
	{
		$this->container = $container;
	}

	/**
	 * Loads a resource.
	 *
	 * @param mixed $resource The resource
	 * @param string|null $type The resource type or null if unknown
	 *
	 * @throws \Exception If something went wrong
	 */
	public function load($resource, $type = null)
	{
		$this->container->setParameter("app.my_custom_loader_parameter", "value");
    
    // Loads services...
	}

	/**
	 * Returns whether this class supports the given resource.
	 *
	 * @param mixed $resource A resource
	 * @param string|null $type The resource type or null if unknown
	 *
	 * @return bool True if this class supports the given resource, false otherwise
	 */
	public function supports($resource, $type = null)
	{
		return ("my_custom_loader" === $resource);
	}
}